The invention relates to a fuel cell unit having at least one fuel cell, to which is connected a cooling circuit associated with a storage unit for storing and providing liquid water coolant to the fuel cell both under normal operating conditions and under frost conditions. Furthermore, the invention relates to a method for operating a fuel cell unit, in which method water coolant stored in a reservoir assigned to a cooling circuit is conveyed to at least one fuel cell by way of the cooling circuit.
As examples of the aforementioned fuel cell units, DE 103 17 123 B4 discloses a fuel cell comprising a cold-start device, and DE 11 2004 002 139 T5 discloses a freeze-tolerant fuel cell power plant. In such fuel cell units, an extended shutdown time and, additionally the action of frost, results in freezing of the water coolant located in a storage unit of the associated cooling circuit. For starting the fuel cell unit it is necessary to melt the frozen water coolant, for which purpose a heating element, particularly in the form of a heater or a heat exchanger, is provided. In this connection, it is important that the holding capacity of the storage unit be adapted to suit the water requirement of the fuel cell unit in normal operating conditions. At the same time, there exists the problem that the process of melting or thawing of the frozen water coolant necessitates the supply of a comparatively great deal of energy and also takes up a comparatively long period of time until the water coolant is thawed and the fuel cell unit can thus be started.
US 2008/0063902 A1 discloses a fuel cell unit in which the water coolant is discharged not only from the fuel cell but also from a reservoir for the water coolant when there is a risk of frost. However, this makes it necessary to refill this reservoir at a later point in time, which basically represents additional outlay.
It is an object of the invention to provide a fuel cell unit and a method for operating a fuel cell unit, which fuel cell unit and method overcome the aforementioned disadvantages and, particularly, enable the fuel cell unit to be started more rapidly following an extended shutdown time and action of frost.
This object is achieved according to the invention by a fuel cell unit having at least one fuel cell, to which is connected a cooling circuit associated with a storage unit for storing and providing liquid water coolant to the fuel cell both under normal operating conditions and under frost conditions. The storage unit is adapted to provide a smaller amount of liquid water coolant under frost conditions than under normal operating conditions. The storage unit is formed with a reservoir that is intended for storing water coolant and is divided into two partial volumes that are separated by a heat-insulating layer
In accordance with the invention, a fuel cell unit has at least one fuel cell, to which is connected a cooling circuit associated with a storage unit for storing and providing liquid water coolant to the fuel cell both under normal operating conditions and under frost conditions. Furthermore, the storage unit is adapted to provide a smaller amount of liquid water coolant under frost conditions than under normal operating conditions. By means of the storage unit of a fuel cell unit formed as noted above, it is possible to provide a smaller amount of liquid water coolant under frost conditions in relation to normal operating conditions. Therefore, only this smaller amount of water coolant must also be converted from its initially frozen state to its liquid state. Accordingly, the process of thawing this smaller amount of liquid water coolant necessitates less energy, and more importantly, also takes up less time. The fuel cell unit of the invention can therefore be started more rapidly under frost conditions than conventional fuel cell units.
It has been discovered that it is not at all necessary, under frost conditions, to provide a large amount of liquid water coolant as is the case under normal operating conditions. This is particularly due to the fact that the associated fuel cell unit having the at least one fuel cell already has a lower temperature under frost conditions than under normal operating conditions. Furthermore, during the start of the fuel cell unit, it is possible to convert additional water coolant from the frozen state to the liquid state and then likewise provide the same to the at least one fuel cell. The waste heat of the at least one fuel cell, already started, can be utilized very preferably for thawing this additional amount of water coolant. Therefore, a smaller amount of external energy is also required on the whole for melting the associated water coolant in a thus formed fuel cell unit of the invention. The remaining frozen water coolant can thus be molten by the warm water produced during the operation of the at least one fuel cell.
In order to enable the same, the storage unit is formed with a reservoir that is intended for storing water coolant and is divided into two partial volumes separated by a heat-insulating layer. In a thus divided reservoir for storing water coolant, it is possible to initially thaw only one of the two partial volumes in order to be able to start the associated fuel cell unit after a short period of time even under frost conditions.
A layer made of polytetrafluoroethylene (PTFE), such as Teflon® is preferably provided as the heat-insulating layer in the storage unit of such type. This type of a layer has a particularly high insulating effect and requires less space.
It is further advantageous to form a through-opening in the heat-insulating layer for the passage of water coolant from one partial volume into the other partial volume. Such a through-opening enables the molten water coolant to flow subsequently from the second partial volume into the first partial volume so that a suction or delivery device for liquid water coolant is required only in the first partial volume.
Furthermore, a heating element is also provided, preferably, in one of the partial volumes for thawing the water coolant located in the associated partial volume, while there is no such heating element located in the other partial volume. In this embodiment, the water coolant in the second partial volume is thawed merely by the liquid water coolant flowing back from the at least one fuel cell. Alternatively, it can be advantageous to provide the second partial volume with a heat exchanger, by which excess heat energy is transferred from the at least one fuel cell into this second partial volume.
The solution of the invention results in a functional improvement of the fuel cell unit since the fuel cell unit of the invention can supply electrical energy and also reach its operating temperature in a shorter period of time. Furthermore, in an appropriate embodiment of the fuel cell unit, less additional energy is required for heating up the water coolant, which has a positive effect on the energy balance of the fuel cell unit. This positive effect is achieved almost without any additional outlay.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.